1. Field of the Invention
The invention relates to a heating apparatus, and a sheet feeding apparatus and an image forming apparatus having heating means for heating sheets.
2. Related Background Art
In an image forming apparatus such as a copying machine or a laser beam printer, design has heretofore been made such that a toner image is formed on a photosensitive drum, and this toner image is transferred to a sheet fed out of a sheet feeding cassette which is sheet stacking means, whereafter the sheet is heated and pressurized in a fixing portion to thereby fix the toner image on the sheet.
In such an image forming apparatus, however, when the sheet absorbs much moisture, that is, when the sheet contains a great deal of moisture therein, the volume resistivity of the sheet falls and therefore, the sheet may sometimes be short-circuited in other grounded portion than a transferring position while the toner image is being transferred. When the sheet is thus short-circuited in other grounded portion than the transferring position, a transferring current does not work well to thereby cause faulty transfer, and this has resulted in the problem that a faulty image occurs. Also, when the sheet absorbs much moisture, there is the problem that wrinkles are liable to occur to the sheet when the sheet passes a fixing roller provided in the fixing portion.
Further, generally, when the sheet absorbs mush moisture, if the sheet is heated and pressurized in the fixing portion, the amount of curl of the sheet becomes great and therefore, after fixing, there occurs the inconvenience that the sheet twines around the fixing roller to thereby cause the clogging of the sheet. Also, when image forming is to be effected on the two sides of the sheet, there occurs the inconvenience that due to curl occurring during image forming on a first side, the sheet twines around a photosensitive drum in the case of image forming on a second side.
So, in order to solve the problems as noted above, it has heretofore been proposed, for example, to install a heating apparatus below the sheet, and heat the sheet by this heating apparatus to thereby remove the moisture of the sheet.
FIG. 8 of the accompanying drawings shows the construction of such a conventional heating apparatus, and this heating apparatus is provided with a heater 80 and a radiation plate 81 on which the heater 80 is mounted. The radiation plate 81 is disposed between the bottom plate 101 of an image forming apparatus and a sheet S with the heater-mounted surface thereof facing down, to thereby heat the sheet S from below. Electric power supply is effected to the heater 80 from a power source contained in the image forming apparatus, or an external power source.
Now, in the conventional image forming apparatus provided with such a heating apparatus, it is often the case that by the recent user's demands for the downsizing and energy saving of the image forming apparatus, the heating apparatus is compelled to be installed below and in proximity to a sheet feeding cassette stacking sheets S thereon, from the viewpoints of space and efficiency.
However, when as described above, the heating apparatus is installed below and in proximity to the sheet feeding cassette, a temperature distribution based on the disposition of a heater intactly affects the surface of the sheets immediately above the radiation plate and as a result, in some cases, the sheets S are locally heated and the humidity distribution and temperature distribution of the sheets S are remarkably partial.
When the humidity distribution and temperature distribution of the sheets S are thus remarkably partial, the sheets S become warily deformed as shown in FIG. 9 of the accompanying drawings, and if in such state, the sheets S are conveyed to a transferring portion, there will arise the inconvenience that faulty transfer is partly caused.
To solve such an inconvenience, it is necessary to dispose the heating apparatus at a location far from the sheet feeding cassette (sheets), and dispose a heater finely and widely so as to be capable of uniformly warming sheets of the largest size, but the adoption of such a construction leads to the bulkiness and high cost of the heating apparatus. Also, the sheets S more readily permit the moisture contained therein to leave off in the portions thereof nearer to the cut end portions thereof and therefore, when the sheets S are simply uniformly heated as described above, there is the undesirable possibility of the moisture remaining more in the central portion of the sheets S.
Also, in a case where the heating apparatus is installed in accordance with the sheets of the largest size, when sheets S of a small size are set, the amount of heat supplied to the sheets S becomes excessively great with a result that not only efficiency becomes worse, but also a temperature rise in an image forming apparatus main body is caused.
In the case of an image forming apparatus for forming a toner image on a photosensitive drum by an electrophotographic process, it is known that the image forming apparatus is liable to be affected by temperature and humidity and particularly, a photosensitive member used as the photosensitive drum has its characteristic affected by temperature and therefore, the evil of an image that the image becomes blurred or stained occurs. Accordingly, when a temperature rise in the image forming apparatus main body is caused as described above and the influence of the temperature rise extends to the photosensitive drum, there is caused the inconvenience that a faulty image also occurs.